Diffraction shield consisting of notched ring which frames passive reflector



y 1964 F. E. ASHBAUGH ETAL 3,140,491

DIFFRACTION SHIELD CONSISTING OF NOTCHEID RING WHICH FRAMES PASSIVEREFLECTOR Filed Jan. 24, 1963 INVENTORS FRED Asf/BAUGH FRANK M Bus/WANAGENT United States Patent This invention relates generally toelectromagnetic wave transmission and reception, and more particularlyto the control of edge diifraction in microwave antennas of theparaboloid type.

In certain applications of microwave antennas it is often necessary tominimize the amount of energy radiated in some directions as Well asmaximize the amount of energy radiated in other directions.

In other applications, such as radio-astronomy and microwave relaylinks, it is very important that the radiation from the back side of aparaboloid antenna be as low as possible. In the field of radioastronomy the main beam of the paraboloid scans the sky to record andmap areas of RF radiation. The back side of the antenna also picks. upsome radiation from the warm earth. This unwanted radiation acts asnoise to the desired signal from the sky, the amount being oftenexpressed as noise temperature. For microwave relay links application,unwanted back radiation results in signal distortion.

The back radiation pattern of a paraboloid antenna is caused by currentsinduced at the outer edge of the paraboloid principally by its primaryfeed. Most attempts to solve the problem have been based primarily onthe reduction of the edge currents by shaping the pattern of the primaryradiator, by shielding the edge of the paraboloid by placing it in atunnel, or by the use of a paraboloid with a very small f/D ratio. Themore recent efforts in this field, such as disclosed in US. Patents2,808,586 and 2,942,265, have been to provide means so that the energyradiated directly from the primary feed arrives at the edgesubstantially out of phase with the energy reflected from the reflector.

The instantinvention operates on the principle of cancellation by phasecontrol and basically consists of a ring of sheet metal positioned onthe periphery of the paraboloid antenna.

Therefore, an object of the invention is to provide means forcontrolling the diffraction of microwaves.

A further object of the invention is to provide means for controllingthe direction of propagation of diffracted electromagnetic waves.

Another object of the invention is to provide means for substantiallyreducing the undesirable effects of edge diffraction in antenna systemsoperating over a broad band of frequencies.

Another object of the invention is to provide means for substantiallyreducing unwanted back radiation in paraboloid antennas.

Other objects of the invention not specifically set forth above willbecome readily apparent from the accompanying description and drawingsin which:

FIG. 1 is a perspective view of the front side of a typical paraboloidantenna with one embodiment of the invention in place;

FIG. 2 is a cross-sectional view taken on the line 22 of FIG. 1; and

FIG. 3 is a cross-sectional view similar to FIG. 2 but showing anotherembodiment of the invention.

A typical paraboloid antenna 1 composed of conductive material andhaving a peripheral flange portion 2 is provided with a ditfractionshield 3. Antenna 1 is provided with a primary feed 5. Shield 3 consistsof a ring "Ice of sheet metal having notches 4 and 4' extending aroundsegments thereof, said notches being spaced at approximately onewave-length intervals and having a depth of approximately /2 wavelength,the wavelengths being relative to the transmission frequency. Thenotched shield causes the electric field at the edge of the parabola tobe broken into two components which appear to be 180 out of phase whenviewed from the rear of the antenna. The result is a broad null in theback radiation pattern.

The theory of the diffraction shield may be better understood byconsidering the view of a paraboloid antenna as shown in FIGURE 1.Energy is radiated by the primary feed 5 toward the antenna shield 3.The electro magnetic field representation is shown in vector form whereI is the poynting vector and represents the power density of the field.The electric vector 1 is shown normal to the plane of the shield 3. Inaccordance with Maxwells equations, both the electric vector E and themagnetic vector E are normal to the flow of energy and are related tothe poynting vector '1? by the vector relationship F=EXFI When theelectromagnetic field reaches the shield 3, it must meet the boundaryconditions as dictated by Maxwells equation. The significant boundarycondition is in this case is defined by the vector relationship J' FXF,where T is the current density induced in the shield 3 in the directionshown in FIGURE 1, and is" is the outward normal to shield 3. It issignificant to note that the induced current in the shield flows acrossthe shield 3.

The outer edge of the shield creates a second boundary condition sincethe current flow must be interrupted. This condition causes the energyentrapped on the shield to be reradiated. The radiation pattern tends tobe cardioid and hence a substantial amount is reradiated to the rear ofthe paraboloid. The notches 4 provide a means of providing anout-of-phase segment. By cutting the notches M 2 deep, the energygenerally reradiated to the rear of the antenna as a result of thecurrent discontinuity at the notch is 180 out of phase from thatreradiated from the outer edge or tooth 4. In order to radiate thenotches must be at least )\/2 wide. On the other hand, the notchesshould not be too wide or too far apart. Considering the notches as anarray, the width and spacing should not be substantially greater than 7\to effect good pattern control. A value of A was chosen for test and wasfound to be adequate.

The following is a description of the operation of the FIG. 1 device:

Consider the rim of the paraboloid of FIG. 1 marked as a compass rose.The linearly polarized primary feed 5 emits an electromagnetic wave inthe direction of the flange portion 2 polarized substantiallyperpendicular to the aperture plane in the 0 (north) and 180 (south)directions and parallel to the aperture plane in the (east) and 270(west) directions. The plane of polarization rotates uniformly fromperpendicular to parallel polarization for directions from 0 (north) to90 (east) and 180 (south) to 270 (west); but rotates from parallel toperpendicular polarization for directions from 90 (east) to 180 (south)and 270 (west) to 0 (north).

Diffraction is associated with only the perpendicularly polarizedcomponent of the electromagnetic wave. For this polarization, a radialcomponent of current is induced in the diiiraction shield 3. Thediscontinuity of this current at the edge of the shield produces are-radiated electromagnetic wave. The current induced in the notch 4 is180 out of phase with the current induced in tooth 4' due to theone-half wavelength difference in distance from the primary feed 5. Asseen from the rear of the paraboloid, the notches and the teeth appearas k) two segmented line sources spaced one-half wavelength apart and180 out of phase. The resulting far-field radiation pattern has a nulldirectly to the rear of the paraboloid. Hence back radiation isvirtually eliminated.

For a linearly polarized primary feed polarized in the north and southdirection, substantial diffraction shielding is provided by only asegment of the shield located at the north and south edges of theparaboloid. Some additional shielding is provided by extending theshield completely around the paraboloid. This also provides shieldingfor dual polarized primary feeds.

FIG. 3 shows another effective type of diffraction shield which consistsof a ring of sheet metal attached to the flange portion 2 of theparaboloid as in FIGS. 1 and 2. However, in this embodiment the outerone-half wavelength is bent back at approximately 30 and is not providedwith notches as the FIGS. 1 and 2 embodiments. The embodiment of FIG. 3provides a partial current discontinuity to the radial current at thepoint of bend. This causes the shield to re-rad-iate at this point. Thereradiatcd energy cannot radiate to the rear of the paraboloid becausethe outer portion of the shield acts as a barrier. Some current is stillpresent at the shield edge and is radiated as before. This radiation issubstantially reduced because of the radiation at the bend.

Based upon the above explained theory, and the well known principle thatradiation will occur at points of current discontinuity, whichdiscontinuity may take on the form of slots, probes, or abrupt changesin directions such as a bend, it was determined that the totaldiffraction pattern is 'a composite of the bend, the length of the bentportion, as well as residual current at the edge of the reflector. Asthe result of such a determination, satisfactory performance wasobtained experimentally with 30 degrees. However, the exact value of 30degrees is not critical to the operation of the FIG. 3 shield.

The embodiments of FIGS. 2 and 3 can also be combined to provide aneffective diffraction shield.

It has thus been shown that the invention provides a simple andeffective means for reducing the radiation from the back side of aparaboloid antenna.

Although particular embodiments of the invention have been illustratedand described, it will be obvious to those skilled in the art thatvarious changes and modifications may be made without departing from theinvention, and it is intended to cover in the appended claims all suchchanges and modifications that come within the true spirit and scope ofthe invention.

What we claim is:

1. A microwave system having means for controlling edge diffractioncomprising: reflector means, ring means extending around at least aportion of said reflector means, said ring means being provided with aplurality of notches.

2. The device defined in claim 1 wherein said notches are spaced atapproximately one wave length intervals and have a radial depth ofapproximately one-half wave length.

3. A microwave system having means for controlling edge diffractioncomprising: reflector means having a reflecting surface, ring meansextending around at least a portion of said reflector means, said ringmeans having an outer edge disposed in a direction backwards withrespect to the outer periphery of said reflecting surface.

4. The device defined in claim 3 wherein the said outer edge isapproximately one-half wave length in radial depth and is disposedbackwards at approximately 30 from the plane of the ring means.

5. A device for controlling edge diffraction of a reflecting meanscomprising: means adapted to be positioned about the edge of at least aportion of a reflecting means, said means including a portion thereofdefining two segmented line sources spaced one-half wave length apartand out of phase.

6. The device defined in claim 5 wherein said portion of said meanscomprises material having a plurality of notches which have a radialdepth of approximately onehalf wave length, said notches being spaced atapproximately one wave length intervals.

7. The device defined in claim 5 wherein said portion of said meanscomprises material having an outer portion disposed in a direction ofapproximately 30 from the plane of said material, said portion having aradial depth of approximately one-half wave length.

References Cited in the file of this patent UNITED STATES PATENTS1,987,780 Latour Jan. 15, 1935 2,460,869 Braden Feb. 8, 1949 2,895,127Padgett July 14, 1959 2,895,131 Butler July 14, 1959 FOREIGN PATENTS726,058 Great Britain Mar. 16, 1955 1,020,065 Germany Nov. 28, 19571,105,354 France June 29, 1954

1. A MICROWAVE SYSTEM HAVING MEANS FOR CONTROLLING EDGE DIFFRACTIONCOMPRISING: REFLECTOR MEANS, RING MEANS EXTENDING AROUND AT LEAST APORTION OF SAID REFLECTOR